The use of retention filters for purifying cooling air, which in itself is obvious, causes very high pressure losses and is therefore unsuitable.
Therefore, to avoid or reduce the risk of blockages, it is known to arrange separators, such as for example cyclones, within the cooling circuit, separating dust or dirt particles from the cooling medium. In these separators, vortices are produced in the cooling medium, causing the dust and dirt particles to be separated from the cooling medium on account of their inertia, and these dust and dirt particles are then carried away via a separate dust discharge opening.
The use of a separator of this type in the form of an axial cyclone is described in DE 198 34 376 A1. The cooling air coming from the compression stage is in this case passed through the axial cyclone before it enters the first guide vane of the turbine stage. A swirl generator is formed in the cyclone, generating a vortex in the cooling air, as a result of which the dust and dirt particles, which have a greater inertia, strike the wall of the cyclone, where they drop downward. At the bottom of the cyclone, they are sucked out via corresponding discharge passages. One drawback of a separator of this type, however, consists in the fact that it takes up considerable additional installation space, which is not always available in turbomachines. Therefore, separators are often used only for applications in which the cooling air can be guided out of the inner region of the turbomachine, purified in the separator outside the inner region and then returned to the inner region in order to cool the corresponding components.
Also known are dust and dirt separators in which the flowing medium, i.e. in this case in particular the cooling air, enters tangentially with respect to a circular or preferably helical flow guide path, where it is guided on a curved path into an inner outflow segment, where the direction of flow is diverted in such a manner that the outflow is axial, substantially normal with respect to the inflow and with respect to the plane of curvature of the circular or helical path. The curved flow of the medium in the guide passage causes dust and dirt particles to be centrifuged toward a radially outer wall of the guide passage. Therefore, dust discharge openings are arranged at the radially outer wall of the flow passage, via which openings a part-stream of the medium discharges the dust and dirt particles in such a manner that the remaining flow, which enters the central outflow segment, is substantially free of impurities in particle or drop form. These separators are of very compact structure on account of the arrangement of the curved guide passage around a central outflow segment. Since the medium is not flowing through retention filters, the overall pressure losses are also low. Separators constructed according to this principle have been disclosed, for example, by GB 2 164 391 and U.S. Pat. No. 3,907,671. However, these separators consume a part-stream of the medium supplied in order to discharge the dust load. In particular in the cooling system of a gas turbine, however, this is compressed air which has been taken away from the process. It is uneconomical and highly undesirable to discard this partial mass stream which has been partially prepared in a complex way.